The long range goal of this research program is to provide information which will serve as a biochemical basis for an understanding of the mental retardation associated with galactosemia and phenylketonuria. Chicks fed high levels of D-galactose have been found to suffer from a neurotoxicity characterized by abnormal energy metabolism. Much of our effort is devoted to comprehending how energy disturbance induced by galactose toxicity is translated into abnormal development of the nervous system. Similarly, we are examining the deficient myelination induced in neonatal rats treated with L-phenylalanine and L-p-chlorophenylalanine.